Brazing and heat treatment of aluminum base alloy castings



United States Patent C) "'ce BRAZIN G AND HEAT TREATMENT OF ALU- MINUM BASE ALLOY CASTINGS Charles H. Dulin, Lakewood, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application June 24, 1955 Serial No. 517,933

8 Claims. (Cl. 148-2131) This invention relates to the brazing of aluminum base alloy castings, especially castings of those aluminumsilicon-magnesium alloys which receive a solution heat treatment to increase their strength and hardness.

Brazed aluminum and aluminum base alloy products have been made for a number of years, but in nearly all cases the structural members of the brazed product have consisted of sheet or other forms of the worked metal. This condition has arisen largely from the fact that the structural members must have a melting point above that of the brazing filler metal and this has meant employing alloys having melting points above 1070 F., the minimum fusion temperature of the binary aluminum-silicon alloys which have been commonly used as the filler metal in making brazed assemblies. Most aluminum base casting alloys have melting points below that of the foregoing aluminum-silicon filler metal or are so close to it that it is not feasible to braze them with this filler metal. In addition to the low melting point, castings often have surface irregularities, an oxide skin and other surface conditions which interfere With proper Wetting of the cast surface by the molten filler metal.

Some brazed assemblies have been made with aluminum base alloy castings of the aluminum-zinc type which do not require solution heat treatment to develop their maximum strength. However, this type of alloy cannot be substituted for the more common compositions because of its poor casting characteristics and loss of strength at elevated temperatures.

One of the best general purpose aluminum casting alloys for both sand and permanent mold casting in respect to casting characteristics and mechanical properties of the cast product is one containing from to 15% silicon and from 0.1 to 3% magnesium, with or without the addition of 0.1 to 1.5% copper. However, it has not been possible to braze castings of these alloys in the ascast condition by employing conventional binary aluminum-silicon alloy filler metals. My invention is directed to utilizing castings of these alloys in making brazed products with aluminum base alloy filler metal, one of the particular objects of the invention being the provision of a method for treating the castings prior to brazing which will permit brazing them at higher temperatures than would be possible otherwise and thus allow the use of certain aluminum base alloys as the filler metal.

My invention is based upon the discovery that a casting of an aluminum base alloy consisting of aluminum, 5 to 15% silicon and 0.1 to 3% magnesium as the essential components can be successfully brazed with aluminum base alloy filler metal up to a temperature of l'060 P. if, before the brazing operation, the casting is heated to between 940 and 1010 F. and held within this temperature range for 'a sufiicient length of time to cause solution of substantially all of the soluble constituents and thereupon cooling to a much lower temperature, preferably no higher than the boiling point of water, either by quenching or by a slower cooling. The heat treated casting may then be assembled with other suitable cast 2,821,495 Patented Jan. 28, 1958 or wrought structural components with a proper filler metal and flux at or adjoining the interface between the parts to be joined and the brazing accomplished in conventional manner. The filler metal must, of course, have a lower melting point than the casting and for this purpose alloys of the aluminum-silicon-copper type, with or without the addition of zinc, may be employed. Other aluminum base alloy filler metals may also be used providing they have a sufficiently low melting point and can be fabricated into the form of wire or sheet. Upon completion of the brazing operation, the bonded region or even the entire assembly is cooled to room temperature. If desired, it may be cooled to the solution heat treating temperature range and quenched to retain the mechanical properties developed by the initial heat treatment.

The preliminary solution heat treatment has been found to increase the temperature at which the casting can be brazed, the increase varying between a few degrees and as much as 50 F. depending upon the composition of the alloy. It has been observed that whereas nonheat treated castings of the composition herein described could not be brazed with commercial aluminum-siliconcopper filler metal they have been successfully brazed after solution heat treatment without distortion or blistering.

The casting alloys adapted to being heat treated and brazed in the foregoing manner should contain silicon and magnesium within the indicated proportions to secure the desired casting characteristics and mechanical properties. in the preferred practice of my invention, from 5 to 10% silicon and from 0.25 to 0.6% magnesium are employed. The castings may be made by the sand, permanent mold or die castings methods according to the usual practices. The strength and hardness of the castings may be increased by the addition of from 0.1 to 1.5% copper to the alloy. It may also be advantageous to include between 0.01 and 1% of one or more of the well' known grain refining and hardening elements of the group composed of boron, titanium, chromium, manganese, zirconium, beryllium and nickel.

The heat treatment of the cast alloy' may be carried out in any conventional type of furnace and under the conditions generally followed in the heat treating art. The atmosphere of an air furnace may be controlled or not as the operator chooses, somewhat better surface conditions being developed where a reducing atmosphere is employed. The castings must be heated to a temperature of at least 940 F. in order to secure solution of the soluble constituents within a reasonable period of time, while on the other hand, the temperature should not exceed 1010 F. to avoid incipient fusion of any low melting point constituents. A soaking period of from 5 to 15 hours is generally sufficient to effect the desired solution. Following the solution treatment the castings may be quenched in an air blast, in cold, warm or boiling water or other liquid to a temperature at or below the boiling point. of water or they may be allowed to cool in still air or its equivalent to the same low temperature range. The drastic or quick chill is desirable if maximum strength is required. Regardless of the manner of cooling the castings, it will be found that the cause of blistering or other manifestations of incipient fusion will have been eliminated.

The filler metal to be used must have a melting point below that of the heat treated castings which eliminates the employment of the well known binary aluminumsilicon alloys. However, alloys can be used which still contain a substantial amount of silicon. It has been found that the combination of aluminum, 10 to 13% of silicon and 2.5 to 4.5% copper is very satisfactory. To obtain a slightly lower melting point without adverse effect, it may be desirable to add from 6 to 16% of zinc.

Broadly, any aluminum base alloy may be used providing it melts and flows at a temperature below the melting point of the heat treated casting, it can be fabricated to wire, strip, sheet or powder form and it has sufiicient strength to serve as a bonding medium.

To make a brazed product, a heat treated casting is assembled with one or more other components, which may consist of castings or wrought products, with filler metal in the form of wire, sheet or strip where a joint is to be established. The filler metal may also be provided in the form of particles in a brazing flux paste or as a layer of sprayed metal at the joint. Instead of the foregoing, the filler metal may be supplied as a layer on or between sheets of rolled aluminum or aluminum base alloys having a melting point above that of the filler metal. Such composite sheet is known in the trade as brazing sheet. A flux must, of course, be provided at the joint in conventional manner. Fluxes suitable for this purpose are disclosed in U. S. Patents 2,299,164 to 2,299,168. The entire assembly may then be heated to the desired brazing temperature below 1060 F. and above the melting point of the filler metal, preferably to a temperature between 1030 and 1060 F., or the filler metal and flux may be locally heated to the brazing temperature by a torch. In either case, the fiux and filler metal are melted and the molten metal fills the space between the members to form a joint. Upon cooling, the filler metal establishes a firm bond between the members.

To retain the tensile properties developed in the castings by the initial solution heat treatment, the joint and adjacent region should be cooled to a temperature between 940 and 1010 F. and then rapidly chilled to a much lower temperature at or near room temperature. If the entire assembly has been heated in a brazing furnace, it should be allowed to cool to 940 F. to 1010" F. either inside or outside the furnace and quenched, preferably in hot water. If retention of the properties of the heat treated castings is not required in all or a portion of the castings, the assembly can be allowed to cool to room temperature in air following the brazing operation.

Although the components of the brazed assembly may consist of heat treated castings, it may also be composed of a heat treated casting and the casting of another alloy or a sheet, plate, extrusion, forging or other wrought form of a suitable aluminum base alloy having a melting point above 1070 F. Examples of such wrought alloys are the aluminum-1.25% manganese or aluminum-1 to 2% magnesium silicide types which have been so successfully employed in the past in the production of brazed assemblies composed entirely of wrought material.

If desired, the brazed assembly may be subsequently artificially age hardened according to customary practice by heating to 300 to 500 F. for a proper length of time, generally from 2 to 12 hours.

My invention is illustrated in the following examples of brazing heat treated castings. Sand cast rectangular /4 inch thick plates of an aluminum-7.0% silicon-0.3% magnesium alloy were solution heat treated 12 hours at 1000 F. and quenched in water at 180 F. A conventional brazing flux in the form of a Water paste was applied to the edge of each plate which was to be joined, a wire of aluminum-10% silicon-4% copper placed at the joint and the plates clamped together in butt joint manner. The assembly was heated to 1055 F. in an air furnace, held long enough to insure fusion of the flux and filler metal, removed from the furnace, cooled to 1000 F. and finally quenched in water at 180 F. The castings showed no sign of blistering or of localized melting. Tensile test bars were cut from the brazed plates across the joint and were found to have an average tensile strength of 17,500 p. s. i. The balance of the plate was aged 3 to 5 hours at 310 F. and tensile test bars cut across the joint had an average tensile strength of 20,500 p. s. i. Non-heat treated sand castings of the same alloy when brazed with the same materials under the same conditions were blistered.

In another case sand cast rectangular inch thick plates of the same alloy were heat treated and quenched in the same manner. The same flux was applied to the edge of each plate, a wire of an aluminum-12% silicon- 3.5% copper-8% zinc alloy placed at the joint, and the plates clamped together in butt joint manner. The assembly was heated to 1035 F. in an air furnace, held long enough to insure fusion of the flux and filler metal, removed from the furnace, cooled to 1000 F. and finally quenched in water at 180 F. The brazed joint was sound and the castings showed no signs of blistering.

Having thus described my invention and certain embodiments thereof, I claim:

1. The method of producing a brazed assembly having as at least one structural component thereof a cast aluminum base alloy consisting essentially of aluminum, 5 to 15% silicon and 0.1 to 3% magnesium comprising heat treating said casting at a temperature between 940 and 1010 F. for a period long enough to cause substantially complete solution of the soluble constituents, thereafter cooling the casting from said solution heat treating temperature to a temperature no higher than the boiling point of water, assemblying said heat treated casting with at least one other structural component with a flux and an aluminum base alloy filler metal provided where the brazed joint is to be formed, said filler metal having a melting point below that of the heat treated casting, heating said filler metal and flux to a temperature between the melting point of the filler metal and 1060 F. but below the melting point of the structural component whereby the flux and filler metal are fused and the molten filler metal establishes a metallic bond between the structural components and thereafter cooling the bonded region to room temperature.

2. The method according to claim 1 wherein the cast aluminum base alloy consists essentially of aluminum, 5 to 15% silicon, 0.1 to 3% magnesium and 0.1 to 1.5% copper.

3. The method of producing a brazed assembly having as at least one structural component thereof a cast aluminum base alloy consisting essentially of aluminum, 5 to 15 silicon and 0.1 to 3% magnesium comprising heat treating said casting at a temperature between 940 and 1010 F. for a period long enough to cause substantially complete solution of the soluble constituents, thereafter quenching the casting from said solution heat treating temperature to a temperature no higher than the boiling point of water, assembling said heat treated and quenched casting with at least one other structural component with a flux and an aluminum base alloy filler metal provided where the brazed joint is to be formed, said filler metal having a melting point below that of the heat treated and quenched casting, heating said filler metal and fiux to a temperature between the melting point of the filler metal and 1060 F. but below the melting point of the structural component whereby the flux and filler metal are fused and the molten filler metal establishes a metallic bond between the structural components and thereafter cooling the assembly to room temperature.

4. The method of producing a brazed assembly having as at least one structural component thereof a cast aluminum base alloy consisting essentially of aluminum, 5 to 15 silicon and 0.1 to 3% magnesium comprising heat treating said casting at a temperature between 940 and 1010 F. for a period long enough to cause substantially complete solution of the soluble constituents, thereafter cooling the casting from said solution heat treating temperature to a temperature no higher than the boiling point of water, assembling said heat treated casting and at least one other structural component with a flux and an aluminum base alloy filler metal provided where the brazed joint is to be formed, said aluminum base alloy filler metal consisting essentially of aluminum, 10 to 13% silicon and 2.5 to 4.5% copper, heating said filler metal and flux to a temperature between the melting point of the filler metal and 1060 F. but below the melting point of the structural component whereby the flux and filler metal are fused and the molten filler metal establishes a metallic bond between the structural components and thereafter cooling the bonded region to room temperature.

5. The method according to claim 4 wherein the aluminum base alloy filler metal consists essentially of aluminum, 10 to 13% silicon, 2.5 to 4.5% copper and 6 to 16% 6. The method of producing a brazed assembly having as at least one structural component thereof a cast aluminum base alloy consisting essentially of aluminum, to 10% silicon and 0.25 to 0.6% magnesium comprising heat treating said casting at a temperature between 940 and 1010 F. for a period of 5 to hours, thereafter quenching the casting from said solution heat treating temperature to a temperature no higher than the boiling point of water, assembling said heat treated and quenched casting with at least one other structural component with a flux and an aluminum base alloy filler metal provided where a brazed joint is to be formed, said filler metal consisting essentially of aluminum, 10 to 13% silicon and 2.5 to 4.5 copper, heating the entire assembly to a temperature between the melting point of the filler metal and 1060 F. but below the melting point of the structural component until the molten filler metal establishes a metallic bond between the structural components, cooling the assembly to a temperature between 940 and 1010 F. and quenching said assembly.

7. The method of producing a brazed assembly having as at least one structural component thereof a cast aluminum base alloy consisting essentially of aluminum, 5 to 15% silicon and 0.1 to 3% magnesium comprising heat treating said casting at a temperature between 940 and 1010 F. for a period long enough to cause substantially complete solution of the soluble constituents, thereafter quenching the casting from said solution heat treating temperature to a temperature no higher than the boiling point of water, assembling said heat treated and quenched casting with at least one other structural component with a flux and aluminum base alloy filler metal provided where a joint is to be formed, said aluminum base alloy having a melting point below that of the heat treated and quenched casting, heating the entire assembly to a temperature between the melting point of the filler metal and 1060 F. but below the melting point of the structural component whereby the flux and filler metal become fused and the molten filler metal establishes a metallic bond between the structural components, cooling the assembly to a temperature between 940 and 1010 F., quenching said assembly, and thereafter aging it at 300 to 500 F. for a period of 2 to 12 hours.

8. The method of producing a brazed assembly having as structural components thereof at least one cast aluminum base alloy consisting essentially of aluminum, 5 to 15% silicon and 0.1 to 3% magnesium and at least one wrought aluminum base alloy component having a melting point not lower than 1070 F., said method comprising heat treating said casting at a temperature between 940 and 1010" F. for a period of 5 to 15 hours, thereafter quenching the casting from said solution heat treating temperature to a temperature no higher than the boiling point of water, assembling said heat treated and quenched casting with said wrought aluminum alloy structural component with a flux and an aluminum base alloy filler metal provided where the brazed joint is to be formed, said filler metal consisting essentially of aluminum, 10 to 13% silicon and 2.5 to 4.5% copper, heating said filler metal and flux to a temperature between the melting point of the filler metal and 1060 F. but below the melting point of the structural component whereby the flux and filler metal are fused and the molten filler metal establishes a metallic bond between the structural components and thereafter cooling the assembly to room temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,602,413 Miller July 8, 1952 FOREIGN PATENTS 122,977 Australia Dec. 2, 1946 

1. THE METHOD OF PRODUCING A BRAZED ASSEMBLY HAVING AS AT LEAST ONE STRUCTURAL COMPONENT THEREOF A CAST ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF ALUMINUM, 5 TO 15% SILICON AND 0.1 TO 3% MAGNESIUM COMPRISING HEAT TREATING SAID CASTING AT A TEMPERATURE BETWEEN 940 AND 1010*F. FOR A PERIOD LONG ENOUGH TO CAUSE SUBSTANTIALLY COMPLETED SOLUTION OF THE SOLUBLE CONSTITUENTS, THEREAFTER COOLING THE CASTING FROM SAID SOLUTION HEAT TREATING TEMPERATURE TO A TEMPERATURE NO HIGHER THAN THE BOILING POINT OF WATER, ASSEMBLYING SAID HEAT TREATED CASTING WITH AT LEAST ONE OTHER STRUCTURAL COMPONENT WITH A FLUX AND AN ALUMINUM BASE ALLOY FILLER METAL PROVIDED WHERE THE BRAZED JOINT IS TO BE FORMED, SAID FILLER METAL HAVING A MELTING POINT BELOW THAT OF THE HEAT TREATED CASTING, HEATING SAID FILLER METAL AND FLUX TO A TEMPERATURE BETWEEN THE MELTING POINT OF THE FILLER METAL AND 1060*F. BUT BELOW THE MELTING POINT OF THE STRUCTURAL COMPONENT WHEREBY THE FLUX AND FILLER METAL ARE FUSED AND THE MOLTEN FILLER METAL ESTABLISES OF A METALLIC BOND BETWEEN THE STRUCTURAL COMPONENTS AND THEREAFTR COOLING THE BONDED REGION TO ROOM TEMPERATURE. 